1. Field of the Invention
The present invention relates to a video processing apparatus, a video processing method, and a computer program and, in particular, to a video processing apparatus, a video processing method, and a computer program for controlling motion blur of a moving picture in a holding type display device such as a liquid-crystal display device.
2. Description of the Related Art
Cathode-ray tubes (CRTs) are in widespread use as a display for displaying a moving picture. Liquid-crystal displays working in a display method different from the CRT are also widely used (see Japanese Patent Application No. 2001-118396, for example).
When a predetermined one of a plurality of frames or fields forming a moving picture is addressed on a CRT, a built-in electron gun successively scans each of horizontal lines (scanning lines) forming the screen of the CRT. The addressed frame or field is thus displayed on the screen of the CRT.
Each of a plurality of pixels forming the addressed frame or field is displayed in an impulsive manner along time axis. In other words, a pixel is displayed at the corresponding location thereof only at the moment the electron gun scans and hits. Display devices adopting the same display method as the CRT are generally referred to as an impulsive type display.
In contrast, liquid-crystal displays hold the display of all liquid crystals forming the entire screen from when a predetermined one of a plurality of frames or fields forming a moving picture is addressed until when the displaying of a next frame or field is addressed. The addressed frame or field is thus displayed on the screen.
It is assumed that one pixel corresponds to a respective liquid crystal. A frame or a field is addressed, and the pixel value of each pixel forming the addressed frame or the addressed field is addressed in the liquid-crystal display device. The liquid-crystal display device applies a voltage, at a level corresponding to the addressed pixel value, to a respective liquid crystal (corresponding to the respective pixel), each pixel forming the screen of the liquid-crystal display device. In response, each liquid crystal emits light at intensity responsive to the level of the applied voltage. Each liquid crystal is continuously supplied with the voltage of the same level and emits light at the same level at least until a next frame or a next field is addressed for displaying. In other words, a pixel having an addressed pixel value is continuously displayed in a respective liquid crystal.
When the pixel value of a predetermined pixel needs to be updated with the next frame or the next field addressed for displaying, the liquid crystal corresponding to the pixel is supplied with the voltage at the level responsive to the updated pixel value (in other words, the applied voltage changes in level). The output level (light intensity) of the corresponding liquid crystal also changes.
The liquid-crystal display device, adopting the display method different from the impulsive type display device such as the CRT, has advantages such as small mounting space requirement, low power consumption, and display relatively free from distortion.
However, the liquid-crystal display device has a drawback that the occurrence of motion blur is more frequent than in the impulsive type display device when a moving picture is displayed.
It has been considered that the generation of motion blur in the liquid-crystal display device is caused by a slow response of the liquid crystal. Image blurring has been considered to take place in the liquid-crystal display device, because it takes time for each liquid crystal to reach an addressed target level (namely, to a level corresponding to the addressed pixel value if one liquid crystal corresponds to a respective pixel).
To overcome this drawback, namely, to control the generation of motion blur, Japanese Patent Application No. 2001-118396 discloses the following technique. In accordance with the disclosed technique, a voltage at a level higher than the level responsive to a target level (namely, to a level corresponding to the addressed pixel value if one liquid crystal corresponds to a respective pixel) is applied. This technique is referred to as an overdrive method, hereinafter. The overdrive method sets, as a target level, a level higher than a normal level, in other words, corrects a target level.
FIG. 1 illustrates the principle of the overdrive method and more specifically, illustrates waveforms in time response of the output level of the liquid crystal with the overdrive method used and unused (normal operation).
As shown, the horizontal axis is time axis, and the vertical axis is an output level of the liquid crystal (intensity of light). A curve 1 represents the waveform of the time response of the output level of the liquid crystal with the overdrive method unused (the normal operation mode). A curve 2 represents the waveform of the time response of the output level of the liquid crystal with the overdrive method used. Here, T represents display time of one frame or one field, namely, time from when one frame or one field is addressed for displaying to when a next frame or a next field is addressed for displaying. Hereinafter, time T is referred to as frame time T or field time T. In the liquid-crystal display device, the frame time T or the field time T is typically 16.6 ms.
As shown in FIG. 1, an output level of a liquid pixel of interest (hereinafter referred to as a target pixel) from among pixels forming the screen of the liquid-crystal display device is a level Yb immediately prior to time zero. When a given frame or field is addressed at time zero, it is assumed that the addressed level of the target liquid crystal (a target level) is a level Ye.
In the ordinary liquid-crystal display device with the overdrive method used, the target liquid crystal is supplied with the voltage at the level corresponding to the target level Ye at time zero. If the target liquid crystal is an ideal one (with response speed at infinity), the output level thereof immediately changes to the target level Ye from the level Yb at the moment the voltage at the level corresponding to the target level Ye is applied. In practice, however, the output level of the target liquid crystal gradually changes from the level Yb to the target level Ye as represented by the curve 1. The response waveform (the waveform of the curve 1) of the output level of the target liquid crystal becomes a delayed waveform.
More specifically, the output level of the target liquid crystal reaches a level Yt1 lower than the target level Ye even at time t1 which is the frame time or the field time T later than time zero (even when the next frame or the next field is addressed for displaying).
It is now assumed that the target level of the target liquid crystal is still the level Ye when the next frame or the next field is addressed at time t1.
In the curve 1 of FIG. 1, the output level of the target liquid crystal gradually rises toward the target level Ye from the level Yt1. Even at time t2 that is the frame time T or the field time T later than time t1 (namely, even when another next frame or another next field is addressed), the output level of the target liquid crystal reaches only a level Yt2 lower than the target level Ye.
In the overdrive method, the target liquid crystal is supplied with a voltage at a level higher than the target level Ye (a level corresponding to a level Ylck as shown in FIG. 1) during a period of time from when one frame or one field is addressed (at time zero in FIG. 1) to when a next frame or a next field is addressed (at time t1 in FIG. 1) so that the output level reaches the target level Ye.
As represented by the curve 2, the output level of the target liquid crystal reaches the target level Ye at time t1 that is the one frame time T or the one field time T later than time zero.
In other words, the target level is modified from the level Ye to the level Ylck higher than the level Ye at time zero in the overdrive method of FIG. 1. The target liquid crystal is supplied with a voltage at the modified target level Ylck. As a result, the output level of the target liquid crystal reaches the unmodified target level Ye (namely, the actually desired level Ye) at time t1 that is one frame time T or one field time T later than application of the voltage.
When the next frame or the next field is addressed at time t1, the target level of the target pixel remains the level Ye in that addressing. Since the output level of the target liquid crystal already reaches the level Ye at time, t1, the target level remains unmodified at the level Ye, and the voltage at the level corresponding to the level Ye is continuously supplied to the target liquid crystal. In this way, the output level of the target liquid crystal is maintained at the target level Ye from time t1 to time t2.
FIG. 2 illustrates a visual change in the output level of the liquid crystal (light intensity) corresponding to the curves of FIG. 1 with the overdrive method in operation and not in operation.
As shown in FIG. 2, the left-hand vertical axis is time axis corresponding to the time axis of FIG. 1. The change in the output level of the liquid crystal with time (the change in the curve 1 of FIG. 1) is shown on the right of the time axis with the overdrive method not in operation. The change in the output level of the liquid crystal with time (the change in the curve 2) is shown on the right hand side FIG. 2 with the overdrive method in operation. As shown in FIG. 2, the output level of the liquid crystal is shown in density of gray tone. The densest gray tone represents the level Yb in FIG. 1, and the lightest gray tone represents the level Ye in FIG. 1.
Even with the overdrive method in operation, the generation of motion blur is not controlled. Currently, no effective method for controlling the motion blur is available in the liquid-crystal display device. The liquid-crystal display device is not free from the above drawback.
The motion blur has been discussed in connection with the liquid-crystal display device. However, this drawback affects not only the liquid-crystal display device, but also any type of display device that includes a plurality of display elements, each of which takes a predetermined time to-reach an output target level from the addressing of the target level, and is associated with at least a portion of a predetermined one of pixels forming a frame or a field.
Many of such display devices adopt a display method in which at least part of display elements forming the screen holds display for a predetermined period of time from the addressing of a predetermined frame or field to the addressing of a next frame or field. Hereinafter, the liquid-crystal display device and the display device adopting such a display method are collectively referred to as a holding type display device. A display state of a display element (a liquid crystal in the liquid-crystal display device) forming the screen of the holding type display device is referred to as a hold display. The above-referenced drawback is a common problem of the holding type display device.